Arsenic is among the elements which are widely distributed in the Earth's crust, where it is present in trace amounts, mainly in the soil and minerals. Under the influence of natural processes and human activities, arsenic is also released to waters and air. The presence of arsenic compounds in drinking water sources poses a threat to human and animal health. The most dramatic effects of the influence of arsenic are observed in Bangladesh and in Western Bengali in India, where, according to the World Health Organization (WHO), over 50 million inhabitants are exposed to the consumption of drinking water contaminated with this toxic element.
Biological removal of arsenic from contaminated areas seems to be a necessary complement to many traditional, chemical methods of remediation. The use of such methods as coagulation or filtration is associated with the removal of not only arsenic, but also other elements present in the treated environment. Current studies on biological systems for arsenic removal, mainly focus on the use of the potential of microorganisms and plants (Kostal et al., 2004, Tripathi et al., 2007).
Effective purification of an arsenic-contaminated waters is associated with the removal of both inorganic forms of arsenic (As III and As V). While arsenates can be efficiently and selectively precipitated on strong adsorbents (Pattanayak et al., 2000), in the case of arsenites there is no possibility of using selective oxidants without side effects to the environment. Microbial oxidation of As (III) becomes therefore an alternative to chemical oxidation. Lievermont et al. (2003) proposed an efficient, low input, two-step technology for arsenic removal from waters with the use of Herminiimonas arsenicoxidans ULPAs1 bacteria. The authors have demonstrated that the strain ULPAs1, immobilised on alginate deposit, can efficiently oxidise even 100 mg/L of As (III) and may be applied in technologies for the removal of arsenic, where initial oxidation of contaminated waters is required.
The known applications of arsenite-oxidising bacteria in bioremediation processes are so far limited to laboratory studies and ex situ methods. The known ways of bioremediation of areas contaminated with arsenic by in situ methods do not fulfill their functions, because bacteria introduced into the “new” environment are not able to survive in the new conditions. This is mainly due to the existence of physico-chemical conditions other than laboratory and to the interspecific competition with the indigenous microflora. The proposed solution to this problem is the biostimulation of indigenous microflora or the use of genetically modified organisms.
Yang et al. (2010) relates to a lab constructed vector, derivative of the plasmid pBBR1MCS-5, carrying genes for the large and small subunits of arsenite oxidase. This vector contains the gene for resistance to gentamicin and its use requires an application of selection pressure of gentamicin at concentration of 60 mg/L. Because of this, an introduction of bacteria harbouring such plasmid into the environment carries the risk of dissemination of genes for gentamicin resistance, and also involves the risk of instability of such strains in the environment. The vector of Yang et al. (2010) is used for constructing strains useful in bioremediation of arsenic, but it only works when introduced into strains originally capable of arsenite oxidation, and it only increases the efficiency of the already existing process. This vector does not cause the acquisition of a new ability, which is the possibility of catalysing the oxidation reaction of As (III) to As (V).
The proposed use of genetically modified organisms involves the introduction of foreign genes carried by them, such as marker genes for antibiotic resistance or encoding the green fluorescent protein (Gfp) into the natural environment, which is unacceptable for social reasons and undesirable for environmental reasons, as well as causing the loss of plasmids in case of the absence of selection pressure for the chosen markers in the natural environment.
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